Derivation of Updated Day to Day Variability Relationships for Major UK Roads
Gerard Lovett, Mott MacDonald, Sansaka Sirivadidurage, Mott MacDonald
The paper describes how updated day-to-day variability (DTDV) relationships were derived using traffic flow and speed data from 2012 for use in Highways England’s MyRIAD software. It also provides comparisons of the new and old DTDV relationships.
Travel time variability consists of two components, namely incident variability and day-to-day variability (DTDV). Highways England commissioned Mott MacDonald to derive new parameters and functions for these two components in order to update the Motorway Reliability Incidents and Delays (MyRIAD) software. This paper describes how updated DTDV parameters were derived.
MyRIAD is used to estimate the journey time reliability impacts of road improvement schemes, including the impacts of DTDV, incident related variability and incident related delays. DTDV accounts for variations in journey time once variability due to incidents and predictable variability are accounted for. Predictable variability includes effects such as seasonal effects, day type effects, within day effects and between link variations. The DTDV functions in MyRIAD are used to assess changes in the variability of journey times by predicting with and without scheme standard deviations of journey times.
This paper describes how DTDV functions were calibrated for eight motorway and dual carriageway road types. This was done using Highways England HATRIS data from 2012. This dataset provided a larger and more comprehensive sample size than was used to derive the previous DTDV curves.
Weighted regression was used to overcome heteroscedasticity. ANCOVA analysis showed that there were significant differences between the DTDV of the eight road types for which data was available. These road types consisted of dual two lane, three lane and 4 lane motorways (D2M, D3M & D4M); three lane motorways with dynamic hard shoulder (D3DHS); three and four lane controlled motorways (D3CM & D4CM); two lane and three lane all-purpose dual carriageways. Consequently, different DTDV relationships were calculated for each of these road types. No evidence was found to justify changing the functional form of DTDV from the standard deviation of average journey time per km as a cubic function of the mean journey time per km.
The DTDV functions derived all exhibited greater variability compared to those previously implemented in MyRIAD.
Association for European Transport